The present invention relates to semiconductor films and a process for their formation. More particularly, semiconductor organic material is blended with a multi-component solvent blend having a combined polarity within a defined range. The blend of semiconductor organic material and multi-component solvent blend is effective for providing a highly ordered semiconductor film having an improved mobility and for providing a device having improved on/off ratio characteristics.
Thin films of organic semiconductor material are used in the production of organic field effect transistors (OFET). The performance of OFETs is measured by parameters such as mobility and depends in part upon the molecular orientation of the semiconductor film. Factors such as interfacial structure, the degree of molecular order and orientation of the thin film affect film properties.
Ordering of the semiconductor film depends in turn on how the thin film is deposited. It is generally believed that increasing the amount of molecular order, for example by reducing the grain boundary resistance, or improving long-range molecular order, permits charge carriers such as electrons or holes to more efficiently move. This can increase the mobility properties of the film.
Deposition techniques that provide inexpensive films of a desired uniformity and thickness do not necessarily provide practical, efficient production techniques or provide semiconducting films that exhibit the requisite mobility and other necessary functional characteristics for thin film transistors. For example, a solvent cast film that is permitted to dry slowly often exhibits a relatively high mobility when incorporated into an OFET. Unfortunately, some deposition techniques that contemplate fast and efficient manufacturing do not readily permit slow evaporation of solvent. For example, though spin coating can yield relatively uniform thin wet films, the solvent usually leaves the film relatively quickly, generally leading to a low degree of molecular order.
Costly processes are required to create highly ordered organic semiconductor films. The effectiveness of organic polymer semiconductor films as active layers in organic film transistors (OFET) depends on the ability of such films to form highly ordered self-assembled layers. These polymers have low solubility in common solvents and dissolve, to a useful extent, in solvents that fall within a narrow range of solvent polarity. This narrows the process window for OFET manufacturing. In addition, the rapid loss of solvent during film formation drives the process away from its thermodynamic equilibrium resulting in poor film structure (e.g. short xcfx80-conjugation length).
In the past, solvents with low polarity were used for the preparation of active layers in OFETs. These solvents evaporated very rapidly during the film formation process due to their high vapor pressure. During the film-forming process, gelation started at the early stages when the solvent began to evaporate. If the solvent had a very high vapor pressure at the process temperature, the final film quality was determined by the morphology and chain conformation present at the very early stage of the process. Such structures have very short xcfx80-conjugation length, which in turn lowers the mobility and affects the overall performance of such films in OFET applications.